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In order to study the change rules of mineral composition, micro-structure evolution characteristics and their relationship with dynamic properties of granite after high temperature treatment, the high temperature tests on Beishan granite in Gansu Province, China, were carried out at 20−1 000 ℃. Microscopic images of granite after high temperature treatment were obtained by slicing technique and polarizing microscope. The evolution of mineral composition, mineral content and micro-structure characteristic parameters of granite with temperature were studied through mineral composition analysis, micro-cracks characteristic identification and parameters calculation. Subsequently, dynamic impact compression tests of high temperature treated granite were carried out by split Hopkinson pressure bar (SHPB) system, to analyze the effects of temperature, impact rate and micro-structure characteristics on dynamic peak stress. On this basis, the quantitative relationship between micro-structure characteristics and macroscopic dynamic properties was established. The results show that high temperature has a significant effect on the mineral composition, micro-structure and impact compressive strength of granite, and 600 ℃ is taken as the threshold temperature. The loading rate also significantly affects the impact compressive strength, and the larger the loading rate is, the less the influence of temperature on the macroscopic dynamic properties is. The evolution process of the micro-structure of granite after high temperature treatment is the internal mechanism for the change of its macroscopic dynamic properties with temperature. The microcrack characteristics have a significant correlation with the dynamic peak stress, and the average crack width is the main factor affecting the dynamic peak stress of granite.

Studying the mechanical behavior of rocks under real-time high-temperature conditions is of great significance for the development of energy caverns, nuclear waste disposal projects, and tunneling engineering. In this study, a real-time high-temperature impact compression test was conducted on Sejila Mountain granite to explore the effects of temperature and external load on its mechanical properties. Based on the concepts of damage mechanics and statistics, a coupled thermal–mechanical (T-M) damage constitutive model was established, which considers the temperature effect and uses the double-shear unified strength as the yield criterion. The parameter expressions were clarified, and the accuracy and applicability of the model were verified by experimental data. The research results indicated that high temperatures had an obvious damaging and deteriorating effect on the strength of the granite, while an increase in impact velocity had an enhancing effect on the strength of the granite. The established constitutive model theoretical curve and test curve showed a high degree of agreement, indicating that the coupled T-M model can objectively represent the evolution process of damage in rocks and the physical meaning of its parameters is clear. [ABSTRACT FROM AUTHOR]

To study the effect of freeze–thaw weathering damage deterioration on the strength of Huashan granite, this paper conducted a freeze–thaw (FT) cycle test on Huashan granite at − 20 to 20 °C and 0–100 cycles. Thus, the basic physical parameters change law of Huashan granite after freeze–thaw weathering are obtained. The SHPB (Split Hopkinson Pressure Bar) device was used to perform uniaxial and confining impact compression of granite, analyze the macro-mechanical test phenomena, and reveal the influence of freeze–thaw damage on rock strength. The results show that frost heave failure is the principal mechanism that causes the strength to decrease after freeze–thaw weathering. The static uniaxial compressive strength and maximum tensile strength of Huashan granite decreased by 38.4 and 21.5% after 100 FT cycles, respectively. The rate of reduction of peak stress does not necessarily grow with the increase of the number of cycles, and the average reduction rate in each phase ranges from 0.1 to 2.4%. It means that the damage accumulation of freeze–thaw weathering on rocks is not a uniformly increasing damage process. The radial restraint of confining pressure can effectively increase the peak stress of the material. When the strain rate is low, no failure occurs in the specimens due to the hoop restraint of confining pressure. [ABSTRACT FROM AUTHOR]

A systemic understanding of the thermal effects on the dynamic behaviour of granite is significant to geological engineering safe applications such as nuclear waste disposal engineering. The experimental granite was taken from Beishan area of Gansu province, China, a preferable region for high-level radioactive waste repository. In order to carry out in-depth research, elevated temperature, confining pressure and impact velocity on the strength of Beishan granite were studied. The results show that above 600°°C, the granite specimens produce a large number of obvious fine cracks, some of which have large cracks, volume expansion and density decrease, and their thermal conductivity decreases rapidly. The minerals change due to heat, resulting in an increase in initial damage, which determines the degree of macroscopic impact compression fracturing, which increases with the temperature under the same impact velocity of uniaxial impact. In triaxial impact compression, the circumferential confining pressure has a strong restraining effect on the radial deformation of the specimen, which effectively improves the mechanical strength of the specimen. Below 600°°C, the peak stress change of granite specimens is relatively small, and the confining effect and impact velocity strengthening effect of confining pressure is stronger than thermal damage effect. Above 600 °C, the peak stress decreases significantly, and the thermal damage effect is stronger than the confining pressure strengthening effect. • Thermal treatment on mechanical properties for Beishan granite is tested. • The temperature damage mechanism is revealed by analysing the physical and mineral properties of Beishan granite. • 600 °C can be considered as the thermal damage threshold of Beishan granite. [ABSTRACT FROM AUTHOR]

In this work, the dynamic compressive mechanical properties of Beishan granite are investigated under the condition of uniaxial load and confined pressure, respectively. The purpose of this investigation is to study the effect of hydrostatic pressure and pre-heating temperature on the strength property and failure of such material. The investigation is carried out with the use of a large-diameter split-Hopkinson pressure bar (SHPB), which is equipped with an active confining device. The dynamic stress–strain behaviors of the granite samples under different confined conditions are obtained with a pre-heating temperature ranging from 25 to 800 °C. We analyzed the temperature-dependent variation of the peak stress, peak strain and elastic modulus of the material under selected values of confined pressure. It is observed that the uniaxial compressive behavior of Beishan granite under unconfined pressure is strongly dependent on the pre-heating temperature. The values of the peak stress and the elastic modulus both decrease as the pre-heating temperature increases, while the value of the peak strain increases with the rise of pre-heating temperature, indicating a characteristic change from elastic-brittle to elastic–plastic. Under confined pressure, the strength of the material can be greatly improved, and higher pre-heating temperature tends to bring down the value of the strength. With the confined configuration, the strengthening of the peak stress is not significantly associated with the value of the confined pressure; instead the dependence on the strain rate becomes more remarkable. For the temperature effect, the value of the peak stress can be linearly correlated with the temperature level through comparative analysis. Moreover, the elastic modulus of the material can be associated with the temperature by a functional form, known as the temperature shift factor. Finally, an elastoplastic-damage constitutive model is established to predict the dynamic mechanical behavior of the material after high-temperature treatment under different confined pressures. [ABSTRACT FROM AUTHOR]